1. Field of the Invention
This invention relates to improved fractional distillation columns and is more particularly concerned with fractional distillation columns employed in the distillation of heat sensitive materials at low pressures .
2. Description of the Prior Art
Fractional distillation columns, conventionally employed in the art for the fractionation of liquid materials on a large scale, generally comprise a vertical column containing a layer of packing, which can take a variety of forms such as glass or porcelain rings, tubes, helices, and the like, and which is the principal means by which intimate contact is ensured between the liquid phase passing down the column and vapor passing up the column.
The preheated, vaporized feed material is introduced into the column generally at a location in the midsection. Vapor passes upwardly through the column and exits from the head, usually via a reflux condenser, while material in the liquid phase passes down the column, countercurrently to ascending vaporized material, and exits at the base where it enters a reboiler from which vaporized material is constantly fed back into the column.
Difficulties are encountered when such columns are applied to the fractional distillation of heat sensitive materials particularly those which have to be distilled under pressure lower than atmospheric. Examples of such materials are aromatic isocyanates such as toluene diisocyanate, diphenylmethane diisocyanate and the like. Thus, for example, the reflux material condensed in head of the column is generally returned, in the conventional type of column, to the packed section via a series of weirs and redistributors which seek to ensure that the liquid be substantially evenly distributed over the surface of the packing. In the passage over the weirs, redistributors and the like, the material is in prolonged contact with the hot surfaces of the devices in question. When the material is heat sensitive, the prolonged contact with the hot surfaces can result in degradation which promotes the formation, and gradual buildup, of deposits. This not only leads to undesirable loss of valuable material, but can result in plugging of the weirs and redistributors necessitating shutdown of the column for cleaning purposes.
Further, unless the redistributors have a very high degree of efficiency, the refluxing material will not be evenly distributed over the packing in its descent down the column. This can result in "dry" spots occurring in the packing with resultant overheating and a tendency for liquid material, which later comes into contact with the overheated spots, to degrade. Again, deposition of solid by-product and clogging of the packing can occur.
The buildup of deposits and clogging or weirs, redistributors, packing and the like in the above manner can also result in development of pressure gradients within the column which can be a serious problem in distillation under extremely low pressure since the performance of the column is drastically reduced.
We have now found that these problems, encountered in the fractional distillation of heat-sensitive materials under high vacuum conditions, can be substantially eliminated by means of a simple but elegant device which we believe to be hitherto unknown in the art in this particular combination.